In line with the continuing downsizing and high density of electronic devices, a structure using three-dimensionally stacked circuit boards, such as module boards, on which electronic components including IC chips and chip components are mounted, has been proposed. (For example, refer to Patent Document 1.) A proposed structure adopts a resin board as an interconnect substrate for connecting circuit boards, and two circuit boards are connected by soldering. This achieves a downsized, shorter, and thinner structure.
However, to cope with the advancement of functions of mobile devices such as mobile phones, the number of connecting terminals between circuit boards is increasing, and thus the terminal pitch of connecting terminals needs to be made finer. This results in reduced soldering area, which in turn causes reduced bonding strength. This leads to connection failures in mobile devices, which are often subject to impact, causing loss of reliability.
To prevent this, a method proposed is to bond and reinforce the circuit boards by applying adhesive resin around an outer side face of the interconnect substrate, after the circuit boards are connected by the interconnect substrate and thermally curing the adhesive resin. However, if the circuit boards re connected with the aforementioned conventional interconnect substrate, and adhesive resin is applied on their outer side face, the adhesive resin may splatter or a connection failure may occur between the circuit boards and interconnect substrate due to thermal expansion of the air in the space enclosed by the two circuit boards and interconnect substrate on heating the structure to cure the adhesive resin. In addition, since the air trapped inside the structure is repeatedly heated after being built into a mobile device, the inside air will repeatedly expand. This may ultimately result in a connection failure between the circuit boards and interconnect substrate, causing loss of reliability. If the viscosity of the adhesive resin is increased to enable application of the adhesive resin in spots so as to prevent air being trapped inside, the adhesive resin may not fully enter the space between the circuit boards, resulting in a smaller bonding area. Accordingly, sufficient adhesive force cannot be secured.
A package assembly structure for a resin-sealed semiconductor device that prevents separation of the bonded area between a case cover and outer case by thermal expansion of air in the sealed area is also proposed. (For example, refer to Patent Document 2.) In this method, sealing resin is applied to two sides of the case cover for bonding so as to prevent separation of the bonded area caused by the increase in pressure inside the case by expansion of air inside the package while curing the sealing resin, which might occur if the entire periphery of the case cover is bonded and sealed with the sealing resin. Using this bonding method, air expanded by heating is released from the remaining two sides to which the sealing resin is not applied. This prevents the increasing of inner pressure, and thus prevents separation of the bonded area.
In a conventional art disclosed in aforementioned Patent Document 1, splattering of adhesive resin or connection failure between the circuit boards and interconnect substrate may occur due to expansion of air inside the area surrounded by the top and bottom circuit boards and interconnect substrate, even if adhesive is applied to the outer side face of the interconnect substrate so as to improve the bonding strength between the circuit boards. In addition, after it is built into a mobile device, repetitive heating causes repeated thermal expansion of the air inside the circuit boards, risking eventual connection failure between circuit boards and interconnect substrate. This degrades the reliability.
In the case of a conventional art disclosed in aforementioned Patent Document 2, partial bonding of the case cover and the outer case in a resin-sealed semiconductor device prevents bonding failure between the case cover and outer case even if the air inside the sealed space is thermally expanded. Application of the adhesive resin may be easy in the case of semiconductor device disclosed. However, if the same method is applied to connection of two circuit boards using an interconnect substrate, the same disadvantage arises. This makes it difficult to reliably secure an area for releasing the air trapped inside the sealed space.
[Patent Document 1] Japanese Patent Unexamined Publication No. 2005-333046
[Patent Document 2] Japanese Patent Unexamined Publication No. H7-14946